In technical practice, it is often necessary to modify the properties of the external and internal surfaces of hollow bodies made of dielectric materials such as polymeric materials, glass, and ceramics in order to obtain the desired surface properties for their end use.
Such modified hollow electrically non-conductive bodies include for example hoses, tubes, bottles, containers, test tubes and cuvettes, hollow fibers and capillaries, catheters and similar hollow instruments and aids used in medicine such as blood bags and similar containers for medical application.
Commercially desirable properties which can be obtained through surface treatment include, for example, hydrophilicity or hydrophobicity, biocompatibility, antimicrobial properties, electrical conductivity, surface permeability for gases and liquids, sorption properties, adhesion, and the like.
For these mentioned surface treatments, so-called “wet methods” are typically currently used. Through the use of liquid solvents and aggressive acidic and alkaline solutions, it is possible to achieve, for example, the cleaning and etching of surfaces, biological decontamination, surface activation, the creation of chemical groups, covering a surface with thin layers of other materials, and surface immobilization of nanoparticles.
According to Advanced Surface Coatings: A Handbook of Surface Engineering, D. S. Rickerby and A. Matthews Blackie & Son Limited, Glasgow and London, England 1991, the application of wet methods of surface treatments are normally associated with environmental problems resulting from the use of toxic and corrosive chemicals.
According to Surface Preparation Techniques for Adhesive Bonding, Raymond F. Wegman, James Van Twisk, Elsevier 2013, an ecologically and often cost-effective alternative to wet surface treatments is the use of non-equilibrium electrical plasma.
An electrical plasma is a sufficiently ionized gas with an approximately equal concentration of positive and negative electrical charge carriers. In technical practice, it is mainly generated through the use of electrical discharges in gases. Electrical discharges are generated in an electrical field sufficient to accelerate electrons to energies of the order of several eV to tens of eV and a corresponding temperature of 104 to 105 K. These values are then sufficient to ionize a gas by collisions of its molecules with electrons. For surface treatment, thermally a nonequilibrium plasma, otherwise also known as non-isothermal plasma, is preferably used, where the electron temperature reaches 104 to 105 K while the temperature of the gas molecules and other electrically neutral particles in the plasma is approximately equal to the ambient temperature.
Diffusion non-equilibrium plasma suitable for surface treatment of materials is simply generated at gas pressures of the order of a thousand times lower than atmospheric pressure. Applications of low-temperature plasma for surface treatment of the external surfaces of hollow bodies are described in documents US20030098085 and JP20080246099. The treatments of the internal surfaces of hollow bodies with a low pressure plasma are described in documents US 2008/0248217, US20100034985, US2013129582, US2013118406 and US2012129582, U.S. Pat. No. 5,521,351 and U.S. Pat. No. 7,967,945 in the international patent application WO/20121097987, and in the article from Xiao Qiong Wen et al.: Vacuum 85(2010) 406-410 and from M. T. Khorasani and H. Mirzadeh: Radiation Physics and Chemistry 76, (2007) 1011-1016.
In J. R. Roth, Industrial Plasma Engineering, Vol. 2: Applications to Nonthermal Plasma Processing, (Inst. of Phys. Publishing, Bristol and Philadelphia, 2001), it is stated that due to the economic and technical demands of pumping and vacuum equipment, the current trend in industrial applications of plasma for surface treatments is the use of techniques allowing for the generation of plasma at atmospheric pressure or at pressures close to atmospheric. As a result of the formation of instability in electrical plasma generated at pressures from approximately 10% of atmospheric pressure, leading to filamentarization, the generation of non-equilibrium diffuse plasma under atmospheric pressure is a known technical problem. In the case of the application of plasma at pressures close to atmospheric pressure for machining internal surfaces of hollow bodies, and especially of long hollow bodies, this technical problem is even more complicated. Another disadvantage is that it usually requires the imposition of metal electrodes or the entire source of plasma to the interior part of these bodies.
According to the results listed in F. Massines et al., J. Phys. D: Appl Phys. 31 (1998) 3411-3420 and in the work of Z. Fang et al., J. Phys. D: Appl. Phys. 42 (2009) 085204 it is known that the diffuse nature of plasma is important for achieving a uniform treatment of the surface, further for increasing the efficiency of the treatment, and for stability of the results obtained.
The article by T. Sato et al., Plasma Process. Polym. 2008, 5, 606-614 describes a method of sterilization of the internal surface of polymeric tubes and catheters using plasma generated at atmospheric pressure. The essence of the method of sterilization involves inserting an internal electrode with a small radius of curvature and the application of a sufficiently high voltage between this electrode and an auxiliary electrode situated on the outside of the tube. This creates an electrical field on the surface of the electrode sufficient to ionize the gas and generate the plasma. The disadvantage of this solution is the technically complicated insertion of the electrode into the interior of the hollow tube and inhomogeneity of the plasma generated in this manner. Another disadvantage of this solution is that it does not allow for the treatment of the external surface of the tube.
The article by M. Polák et al. Plasm. proc. Polym. 2012, 9, 67-76 and the international patent application PCT/EP2011/0510 both describe a method for generating plasma under atmospheric pressure inside a long tube using a pair of electrodes spirally deposited on the surface of the tube. The disadvantages of this solution are the technically complicated construction of the electrodes themselves and their complicated electrical insulation. Another technical disadvantage of this solution is that it is necessary to reduce the electrical voltage needed to ignite the electrical discharge without the danger of disturbing the electrical insulation between the electrodes. This is achieved using expensive inert gases such as argon (Ar) and helium (He). Another disadvantage of this solution is that it does not allow for the treatment of the external surface of the tube.
The document US20130189156 describes a solution using a dielectric barrier discharge to sterilize the internal surface of hollow containers using two enclosed external electrodes. The disadvantage of this solution, as in the similar solution described in A. Schwabedissen et al, Contrib. Plasma Phys. 47 (2007) 551-558 using a surface barrier discharge, is the very inhomogeneous treatment of the internal surface of the hollow body.
Basic properties, electrode geometry, and method of powering the surface dielectric barrier discharge generated using metal electrodes are described in the document WO2008082297 and in the publications of V. I. Gibalov and G. J. Pietsch J. Phys. D: Appl. Phys. 33 (2000) 2618-2636 and James M. Williamson at al.: J. Phys. D.: Appl. Phys. 39 (2006) 4400. The disadvantages of this solution can be the impossibility of the treatment of the internal surface of the hollow bodies and the use of metal electrodes, the corrosion, erosion, or sputtering of which can lead to their inoperativeness and contamination of the surface.
In James M. Williamson at al.: J. Phys. D: Appl. Phys. 39 (2006) 4400 a surface dielectric barrier discharge was studied, generated using AC voltage pulse and pulse voltage with the rapid increase of the voltage pulse. It was observed that when using AC voltage, the surface dielectric barrier discharge has a filamentary structure. It was found that the creation of a diffusion plasma is possible only with the use of a rapidly rising voltage pulse, which is technically very demanding.
The objective of the invention is to create a method of plasma treatment of the internal and/or external surface of a hollow electrically non-conductive body which would eliminate the aforementioned drawbacks, in particular which would reduce or limit the possibility of damaging the electrodes, and which would ensure the uniform and easy treatment of hollow electrically non-conductive bodies on their internal and external surfaces. Another objective of the invention is to create a device for implementing this method.